The present invention relates to system and method for controlling the shifts of an automatic transmission for vehicles and, more particularly, to a technology for controlling the shifts of an automatic transmission which has first and second transmission assemblies connected in tandem for executing the shifting operations independently of each other.
Generally speaking, an automatic transmission is equipped with a gear transmission assembly and a plurality of frictional engagement means so that the combinations of the engagement states of the plural frictional engagement means can be switched to achieve any of a plurality of gear stages by actuating hydraulic control means.
In the prior art, the number of gear stages to be achieved by the automatic transmission is ordinarily three or four. In recent years, however, there has been noted a multistage automatic transmission which can achieve five or six or more gear stages so as to improve the power performance and the fuel consumption better. If, for example, a first transmission assembly capable of switching three stages of 1st to 3rd speeds and a second transmission assembly capable of switching high and low two stages are connected in tandem, there is provided an automatic transmission which can achieve forward six gear stages, as disclosed in Japanese Patent Laid-Open No. 64-15560.
If, however, this example is enabled to achieve all of those six gear stages according to a predetermined shift pattern of one kind, the shifts are seriously frequently executed to cause a problem that the driver cannot be kept away from a busy shift feel.
In case of a shift from the 2nd to 3rd speeds, for example, the automatic transmission disclosed in the above-specified Laid-Open has its first transmission assembly shifted to a high gear and its second transmission assembly shifted reversely to a low gear so that the automatic transmission is shifted up in its entirety. As a result, the two transmission assemblies have to be simultaneously shifted while being precisely synchronized, to cause another problem that the shift control is seriously difficult.
In case, moreover, the accelerator pedal is abruptly released, the disclosed automatic transmission is caused to establish a jumping shift from 2nd to 5th speeds. In this case, the first transmission assembly has to execute an upshift from the 1st to 3rd speeds whereas the second transmission assembly has to execute a downshift. Then, three frictional engagement means have to be simultaneously applied, thus causing a further problem that the smooth shift becomes more difficult to execute.
In order to solve the aforementioned various problems, the invention disclosed in Japanese Patent Laid-Open No. 61-244957 is inhibited to execute the shifts as a whole of the automatic transmission by shifting the first and second transmission assemblies simultaneously.
If, however, all the shifts, in which the first and second transmission assemblies are to be simultaneously shifted, are merely inhibited, the merit of the multiple stages can be hardly exploited to make it impossible to avoid the problem of a deterioration in the running performance such as a reduction in the driving force.
In order to establish a high drive torque in a high speed range or improve the effect of the engine braking, the automatic transmission has its shift ranges determined by inhibiting an upshift to a predetermined or higher gear stage. The ranges conceivable to be held are the L-range, the 2nd range and 3rd range, which are manually switched.
In case of a shift from a range of holding a lower gear stage to a range of allowing a higher gear stage, the upshift is caused by switching the ranges if the running state requires a higher gear stage. For example, in case the 2nd range is selected in the automatic transmission of the prior art and is switched from the 3rd range holding the 2nd speed to the D range allowing the highest gear stage, the possibility of causing the upshift from 2nd to 3rd speeds is remarkably high. Moreover, this upshift from the 2nd to 3rd speeds is achieved by shifting the first and second transmission assemblies in the opposite directions, and the region (to be determined by the vehicle speed and the engine load) for causing the upshift is drastically widened. Despite this fact, however, the varying ranges of the engine torque and the speed of rotary components are different depending upon the variations and the varying rate of the throttle opening so that the engagement pressures of the frictional engagement means for setting the gear stages are accordingly different. This makes it seriously difficult to achieve the shift from the 2nd to 3rd speeds with low shift shocks all over the aforementioned wide ranges.